The present invention relates generally to a catheter, and more particularly, to a catheter having a miniature x-ray generator unit at its tip for providing a biologically effective dose of x-ray radiation.
X-ray radiation having a biologically effective spectrum, for example, having an energy in a range of about 10 keV to about 40 keV, can be utilized for a variety of medical treatments. For example, such x-ray radiation can be employed for preventing restenosis in blood vessels that have undergone angioplasty and/or it can be employed in some oncological procedures, such as interstitial radiosurgery of tumors.
Miniature x-ray generators that produce biologically effective radiation, and that can be deployed in proximity of a target tissue by utilizing flexible catheters, are known. Such x-ray generators provide advantages over radioactive isotopes, such as, 90Sr, 32P, or 192Ir, as sources of x-ray radiation. For example, unlike the radioactive isotopes, the energy spectrum and/or the dose rate provided by these x-ray generators can be varied over a relatively wide range.
A conventional approach to powering a miniature x-ray generator disposed in a catheter utilizes an external power supply to generate a high voltage, e.g., in a range of about 10 to 40 kV, required by the x-ray generator, and transmits the voltage to the generator via a small-diameter electrical cable that extends from the power supply through the catheter to the generator.
This approach has a number of disadvantages. For example, the thickness of an insulation layer needed to insulate the electrical cable can adversely affect the flexibility of the cable, and consequently maneuverability of the catheter. Further, there is a danger of insulation breakdown, particularly, during a medical procedure. Such insulation failure can at the least require the withdrawal of the catheter and the x-ray tube, or more ominously, it can expose the patient or medical personnel to electrical shock.
Accordingly, there is a need for a catheter having an x-ray generator which provides enhanced operational safety, ease of construction, and better flexibility.
The present invention provides a catheter having a flexible body that extends from its proximal end to its distal end. The catheter further includes an x-ray generator disposed in the distal end region of the flexible body for generating biologically effective x-ray radiation, for example, x-ray radiation having an intensity in a wavelength range that is effective for treating a patient""s tissue. A miniature transformer is also disposed in the distal end region of the flexible body, and is electrically coupled to the x-ray generator, to power the generator. More particularly, the transformer includes a primary winding that receives an AC input voltage having a root-mean-square (rms) amplitude, for example, in a range of about 100 V to about 4 kV, and further includes a secondary winding that applies an AC output voltage having an rms amplitude, for example, in a range of about 10 kV to about 40 kV, to the x-ray generator. The incorporation of the miniature transformer at the distal end of the catheter eliminates the need for a high voltage transmission line along the entire length of the instrument and greatly reduces the insulation needed within the body of the catheter.
In one aspect, the x-ray generator has a length that is less than about 30 millimeters, and a maximum cross-sectional dimension, for example, a diameter when the cross-section is circular, that is equal or less than approximately 3 millimeters. Further, the transformer can have a length that is less than about 50 millimeters and a maximum cross-sectional dimension that is equal or less than approximately 3 millimeters. The small dimensions of the x-ray generator and the transformer advantageously allow their coupling to the tip of a catheter having an outer diameter with a dimension of a few millimeters to be deployed, for example, in a patient""s artery to irradiate a selected tissue target.
The x-ray generator and the transformer can be formed as a monolithic device. Alternatively, the x-ray generator and the transformer can be formed as separate devices that are mechanically and electrically coupled to one another.
In further aspects, a catheter of the invention includes a flexible electrical cable, having a diameter in a range of about one millimeter to about two millimeters, that extends from the proximal end to the distal end of the catheter body. The electrical cable transmits an AC input voltage from an AC source, for example, an AC power converter, to the primary winding of the transformer. The electrical cable can be, for example, in the form of a coaxial cable having a pair of elongated coaxial conductors, one of which is electrically grounded and the other carries an AC electrical voltage to the primary winding of the transformer. An insulating inner layer having a thickness in a range of about 0.01 mm to about 0.2 mm insulates the two conductors from one another. In addition, an outer insulating layer having a thickness in a range of about 0.001 mm to about 0.2 mm provides an insulating cover for the cable. The insulting layers can be formed of a variety of materials, such as, polyethylene or Teflon(trademark). The inner insulating layer is preferably selected to be able to withstand a voltage differential in a range of about 100 V to about 4 kV.
In another aspect, the x-ray generator can generate radiation having an energy in a range of about 10 keV to about 40 keV, and provides an x-ray output power in a range of about 1 mW to about 100 mW. The x-ray generator can include an evacuated housing and a cathode that is disposed in that housing. The cathode is preferably formed of a metal, such as tungsten, and is electrically coupled to the secondary winding of the transformer to receive a voltage in a range of about 10 kV to about 40 kV therefrom. Other methods, such as, carbon nanotube or micro-machined silicon pyramid, can also be utilized for forming the cathode. The x-ray generator can further include an anode separated from the cathode by a selected distance. The anode is preferably formed of a high-Z refractory metal, such as tungsten, and can be electrically grounded so as to create an electrical potential difference between the cathode and the anode, thereby generating an electric field therebetween. The cathode emits electrons in response to application of a voltage thereto, for example, during a negative portion of each cycle of an AC voltage. The electric field between the cathode and the anode accelerates these electrons to the anode, and the impact of the electrons with the anode effects production of x-ray radiation.
In further aspects, the x-ray generator can include a window that is substantially transparent to the x-ray radiation to facilitate transmission of the generated radiation to the outside environment. The window has preferably a transmission coefficient of approximately 99% or higher for x-ray radiation having an energy in a range of about 10 keV to about 40 keV. In one embodiment, the window is formed of beryllium and has a thickness in a range of approximately 10 microns to approximately 100 microns. The window can be formed of a sheet of a material having a substantially uniform thickness, and can be directly coupled to the housing of the x-ray generator, for example, in an opening provided in the housing. Alternatively, the window can be supported by a mesh, which is in turn mechanically coupled to the x-ray generator""s housing.
In a related aspect, the x-ray generator can include an extraction electrode disposed in the housing between the cathode and the anode, and maintained at an electrical potential intermediate the potential difference between the cathode and the anode. The extraction electrode can advantageously control emission of electrons from the cathode, and can further focus the emitted electrons onto the anode.
In other aspects, the transformer can include a core that has preferably a cylindrical shape, and is formed of a ferromagnetic material, such as iron or a ferrite composed of oxides of Fe, Ni, Mn, and Zn. The cylindrical core can have a diameter, for example, in a range of about 0.1 mm to about 2 mm, and a length in a range of about 5 mm to about 30 mm. Further, the primary and the secondary windings of the transformer can be formed of coils, constructed of a conductive metal, such as copper wire. The low current, e.g., in a range of about 10 to 100 microamperes, flowing through the secondary windings, and proportionally higher through the primary winding, while the transformer is operational allows utilizing small-diameter wire for the construction of the windings. For example, in one embodiment, copper wire having a diameter in a range of approximately 0.025 mm to approximately 0.1 mm is employed for constructing the secondary winding, and copper wire having a diameter in a range of approximately 0.1 mm to 0.6 mm is employed for constructing the primary winding. Those skilled in the art will appreciate that conductive wires formed of other materials and/or having other diameters or shapes other than circular (e.g., oval or rectangular) can also be utilized for forming the transformer windings so long as the electrical resistance of the wiring is sufficiently low to allow the passage of the requisite currents through the windings without a high degree of heat generation and/or unduly increasing the size of the transformer.
In a related aspect, the transformer can include two secondary windings, one of which applies an AC voltage to the cathode of the x-ray generator, and the other applies an AC voltage to the extraction electrode. Alternatively, the transformer includes one secondary winding having a primary tap for applying a voltage to the cathode, and a secondary tap for applying a voltage to the extraction electrode.
In another aspect, the core, the primary and the secondary windings of the transformer can be xe2x80x9cpottedxe2x80x9d in an insulating material, such as, polyethylene, silicone, epoxy, or polyurethane that electrically insulates these transformer components from the housing. The insulation layer covering these transformer components can preferably withstand voltage differences of approximately 40 kV or higher.
In another aspect, a catheter of the invention includes a miniature x-ray generator having a plurality of cathodes and anodes. The multiple cathodes and anodes can be utilized, for example, to ensure that at least one cathode emits electrons during each of the negative and positive swings of the AC voltage of the secondary winding of the transformer, thereby enhancing the efficiency of x-ray generation.